1. Field of the Invention
The present invention relates to a method, system, and program for copying tracks between a primary storage and secondary storage.
2. Description of the Related Art
Data storage systems may maintain a secondary copy of data at a remote site to use in the event of a failure at the primary site. Such dual or shadow copies are typically made as the application system is writing new data to a primary storage device. International Business Machines Corporation (IBM®), the assignee of the subject patent application, provides two systems for maintaining remote copies of data at a secondary site, extended remote copy (XRC) and peer-to-peer remote copy (PPRC). These systems provide a method for recovering data updates between a last, safe backup and a system failure. Such data shadowing systems can also provide an additional remote copy for non-recovery purposes, such as local access at a remote site. These IBM XRC and PPRC systems are described in IBM publication “Remote Copy: Administrator's Guide and Reference,” IBM document no. SC35-0169-02 (IBM Copyright 1994, 1996), which publication is incorporated herein by reference in its entirety.
In such backup systems, data is maintained in volume pairs. A volume pair is comprised of a volume in a primary storage device and a corresponding volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. Typically, the primary volume of the pair will be maintained in a primary direct access storage device (DASD) and the secondary volume of the pair is maintained in a secondary DASD shadowing the data on the primary DASD. A primary storage controller may be provided to control access to the primary storage and a secondary storage controller may be provided to control access to the secondary storage.
In PPRC mirroring, host updates may be copied synchronously or asynchronously. If the host writes the updates synchronously, then the primary storage controller does not return acknowledgment of the write until the write completes at the secondary site, and acknowledgment is returned to the primary controller. If the host writes the updates asynchronously, then the primary storage controller returns the write acknowledgment immediately even before the data is physically-copied to the secondary storage.
Synchronous writing provides greater data security because the host does not continue until the host is ensured that the data has been applied at the secondary site in correct order. However, the delays in returning acknowledgment to the host required for synchronous remote copying may affect the operation of application programs accessing the host system waiting for write complete. Although the asynchronous copy mode may return acknowledgment to the host significantly faster than synchronous, the host still cannot be assured that the data has been applied in proper sequence at the secondary site, especially if data is lost during transmission through the network or fabric, such as a Storage Area Network (SAN).